Stepper motors, or rotary transducers, are used in a wide variety of products where it is important to control the rate of rotation and rotational position of a shaft or other device.
An example of a prior art stepper motor or selectively positionable rotary transducer is disclosed in U.S. Pat. No. 4,358,694 to Grundland. The stepper motor disclosed in Grundland discloses a ring gear including at least one permanent magnet and a rotor which is rotationally moveable relative to the ring. An odd number of poles are provided on the rotor for selectively providing magnetic force to cause relative rotation therebetween. The magnet has a precisely machined inner wear surface which is rotated against the outer surface of the fixed stator. Teflon or oil may be used to reduce friction between the magnet and the stator but are not intended to interfere with the existence of an air gap between the stator and the rotor.
Other approaches to manufacturing stepper motors include the use of permanent magnets or rare earth materials. Such materials add cost to stepper motor construction.
Prior art stepper motors tend to be fairly large in size and limited in power output.
Any air gap between the opposite poles in the magnetic circuit results in a reduction of holding torque of the motor. The attractive forces between opposing poles is significantly reduced as a result of the existence of an air gap in the magnetic circuit.
These and other disadvantages encountered by the prior art are addressed by the present invention as summarized below.